1. Field of the Invention
This invention relates to a method for preventing dissolution of Group VIII supported noble metal catalysts in acidic environments.
2. Prior Art
An undesirable side effect in many liquid phase catalytic syntheses employing a supported catalyst of a Group VIII noble metal is that the noble metals tend to dissolve when in media which are "corrosive," that is, provide an oxidizing environment. Corrosive media or environments include liquids which contain an oxidizing acid, particularly those containing HCl, H.sub.2 SO.sub.4 and/or HNO.sub.3, even in very low concentrations. Liquid media subjected to treatment with oxygen and containing any acid are corrosive, as are those containing any acid plus H.sub.2 O.sub.2 or any other oxidizing agent.
The corrosion or dissolution reaction can be represented by the equation EQU M.revreaction.M.sup.N.spsp.+ +ne.sup.-
in which M is a Group VIII noble metal which is oxidized to an N.sup.+ valence state with loss of n electrons. The reverse reaction represents reduction of the soluble noble metal compound to the metal.
Typical of processes in which losses by solubilization of Group VIII noble metals from supported catalysts become especially troublesome are liquid phase catalytic processes for producing hydrogen peroxide from its elements, employing supported precious metal catalysts, e.g., from Groups I or VIII of the Periodic Table, as proposed by Hooper in U.S. Pat. Nos. 3,336,112 and 3,361,533, herein incorporated by reference. The liquid media described in these references contain a non-acidic oxygenated organic compound and at least one strong acid, e.g., H.sub.2 SO.sub.4, HNO.sub.3, HF, HCl, HBr, H.sub.3 PO.sub.4 or sulfonic acids, in concentrations ranging from 0.01 N to 2 N.
In this type of synthesis, the combination of hydrogen peroxide and/or oxygen and one or more strong acids, particularly hydrochloric acid required to attain reasonable levels of hydrogen peroxide, provides an oxidatively active environment which leads to serious losses of palladium or other catalytic metals by dissolution.
In a representative case, deactivation of palladium on carbon catalyst used in batch synthesis of hydrogen peroxide from its elements appears to reach a maximum after about 3 hours' reaction. The apparent decline in soluble palladium as a function of time is attributed to the redeposition and/or readsorption of palladium on carbon. It will be understood that loss of Group VIII metal from the catalyst owing to mechanical attrition will also occur.
In a typical continuous process for the synthesis of hydrogen peroxide, employing a bed of palladium on carbon catalyst, the cumulative loss of palladium was 16% after 185 hours of operation.
Loss of palladium or other Group VIII noble metals is an economically unacceptable occurrence due to (1) the loss of expensive palladium, (2) the resultant decrease in catalyst activity from dissolution losses and catalyst deactivation via redeposition of soluble palladium and to (3) the contamination of the product. Although catalyst loss can be reduced somewhat by physical means, no process previously available is capable of stopping the catalyst dissolution reaction.
Cathodic protection has been utilized to prevent or minimize corrosion of marco-continuous metal surfaces, such as bridges, ships or storage tanks, by sea water or other saline media, but had not, prior to the instant invention, been employed to prevent dissolution of Group VIII noble metals from supported catalysts used in oxidizing environments. This technique has been discussed in detail by M. Stern, "Principles of Cathodic Protection," in Symposium on Corrosion Fundamentals, A. S. Brasunas et al, editors, University of Tennessee Press, Knoxville (1956). Basically, the concept is based on two observations:
i. Metal corrosion is typically an oxidation process characterized by a reversible equilibrium potential when a corrodible metal is placed in contact with a corrosive medium or electrolyte. In the case of palladium, the potential is -0.620 volts. In a galvanic arrangement, corrosion occurs at the anode.
ii. Each corroding system has a characteristic corrosion potential and current, which are measured by anodic and cathodic polarization curves.
Electroplating of the platinum group metals, specifically of platinum, palladium and rhodium, from ammoniacal media has been disclosed by Keitel et al in U.S. Pat. No. 1,779,436.